Move Singleton and related structs to namespace base

chrome/browser/history/top_sites_factory.h

Index: chrome/browser/history/top_sites_factory.h
diff --git a/chrome/browser/history/top_sites_factory.h b/chrome/browser/history/top_sites_factory.h
index 8794a11dd070b5f439558998f00f8e923303caef..e5618346b2fba4109da7319302764b92dc89d072 100644
--- a/chrome/browser/history/top_sites_factory.h
+++ b/chrome/browser/history/top_sites_factory.h
@@ -12,8 +12,10 @@
 
 class Profile;
 
+namespace base {
 template <typename T>
 struct DefaultSingletonTraits;
+}
 
 namespace history {
 struct PrepopulatedPage;
@@ -39,7 +41,7 @@ class TopSitesFactory : public RefcountedBrowserContextKeyedServiceFactory {
       const std::vector<history::PrepopulatedPage>& prepopulated_page_list);
 
  private:
-  friend struct DefaultSingletonTraits<TopSitesFactory>;
+  friend struct base::DefaultSingletonTraits<TopSitesFactory>;
 
   TopSitesFactory();
   ~TopSitesFactory() override;